A web validation aphorism
There's a famous folk-rule on Usenet and mailing lists to the effect that a spelling flame usually has at least one spelling mistake itself. There seems to be a related rule on the web, which I will put this way:
Websites that boast about validating often don't.
(The corollary is that a certain amount of people agitating for valid web sites don't have valid web sites.)
Now, HTML validation is certainly picky (arguably more picky than spelling), but I'd like to think that people who care enough to stick a badge on their website care enough to run things through a validator. Apparently not, though. (And agitating for web standards while not following them is just ironic.)
One possible reason for the problem is that websites change, creating opportunities for validation errors to creep in. And certainly HTML is hard to write by hand; I suspect that few websites are created with tools that guarantee validation all the time.
(While WanderingThoughts usually validates, this is mostly because its HTML is automatically generated and thus any invalid bits are generally the sign of a programming error that I want to step on. And it only validates to HTML 4.01 transitional, a relatively loose standard to aim for.)
Sidebar: an extreme of valid HTML
Not only is valid HTML picky, it's tricky. For example, here is a rather extreme example of perfectly valid but quite twisted minimal HTML. It's startling how much markup you can leave out and still be legal.